Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in making industrial and building products, particularly gypsum wallboard. Gypsum is a plentiful and generally inexpensive raw material which, through a process of dehydration (or calcination) and rehydration, can be cast, molded or otherwise formed into useful shapes. The base material from which gypsum wallboard and other gypsum products are manufactured is the hemihydrate form of calcium sulfate (CaSO4·½H2O), commonly termed “stucco,” which is produced by heat conversion of the dihydrate form of calcium sulfate (CaSO4·2H2O), from which 1½ water molecules have been removed. After rehydration, the hemihydrate dissolves, gypsum crystals precipitate, and the crystal mass sets and becomes solid, providing a set gypsum material.
In order to make gypsum-containing products, gypsum-containing slurries are generally prepared. The gypsum-containing slurries may contain stucco and water, and other known ingredients and additives, such as, for example, starch, dispersants, accelerators, binders, glass fibers, and paper fibers, etc.
Starch is known to be a good binder for gypsum crystals within a set gypsum core, and can increase board strength. In addition, starch can function to provide a better interface between the set gypsum core and a cover sheet in a wallboard product.
Use of a fine ground pregelatinized corn starch (for example, where >99% of particles pass through a 100 mesh screen), in gypsum-containing slurries is known in the art. However, the fine ground starch produces a fine dust that is not easy to handle. Another problem is that the fine starch can require additional process water. For example, one pound of dry fine ground starch can increase water demand by about 10 lb/MSF. (MSF is an industry term for 1,000 square feet.)
Coarse starch, that is larger particle size starch, can be used in the gypsum-containing slurry, since it is less dusty and easier to handle. In addition, larger particle sizes require less water in the slurry. However, due to a larger particle size it is difficult to fully disperse coarse starch in the gypsum slurry using the standard dry basis process, such as during very short-term mixing with water and stucco through a board mixer where contact time is typically less than one second. If a way could be found to effectively use a larger particle size pregelatinized starch, water usage could be reduced and cost savings realized. Additionally, the use of a larger particle size pregelatinized starch could increase board strength, and thus serve as a useful contribution to the art.